Photopolymer imaging from solvent ink film images

ABSTRACT

Negatives, and laminates containing negatives, for exposing liquid photopolymers in UV flexographic plate making are created by printing opaque images directly onto the protective or release cover films used in plate making processes. Combining negatives and cover films in this way eliminates two layers and two interfaces in the exposure process, reducing cost and improving quality. By bringing the negative images as close as possible to the liquid photopolymer, printed dots of “digital quality” are created.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 14/027,760 filed Sep. 16, 2013, which is a Continuation-In-Partof U.S. patent application Ser. No. 13/618,507 filed Sep. 14, 2012, andwhich in turn is a Continuation-In-Part of U.S. patent application Ser.No. 12/895,561 filed Sep. 30, 2010, claims priority thereto, andincorporates these into the instant application in their entirety. Arelated patent application is U.S. patent application Ser. No.13/902,301 filed Feb. 19, 2013. These applications are owned in commonand have at least one inventor in common, and are incorporated into theinstant application in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO A BIOLOGICAL SEQUENCE LISTING

Not applicable.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention is in the field of printing, more specifically in thefield of making flexographic printing plates with digital quality flattop dots using liquid photopolymers, and still more specifically in thefield of using solvent-based inks to print the flexographic platenegatives.

2. Description of the Related Art

A prior art method of preparation of a simple flexographic (“flexo”)printing plate from cast liquid photopolymer currently involves thesteps of (a) printing a black negative image on a white substrate; (b)photographing the negative image; (c) developing the film negative; (d)positioning the film negative on the bottom glass of a UV exposure unit;(e) positioning liquid dams and taping the negative in place withmasking tape as necessary; (f) placing a thin plastic vacuum sheet(cover film) over the negative; (g) casting liquid photopolymer over thefilm to the proper depth; (h) applying a vacuum to the laminate thusformed (negative, vacuum sheet, and photopolymer); (i) exposing thislaminate to UV (or other appropriate actinic) radiation through thenegative for an amount of time sufficient to create a cross-linkedpolymerized image in the photopolymer; (j) removing the laminate fromthe unit and separating the vacuum sheet and negative from thecross-linked photopolymer; and (k) removing the unexposed material todevelop a relief image.

The vacuum step (h) is important. If there are any pockets of airbetween the negative and the adjacent surface of the photopolymer, theUV light will be refracted by the interfaces between the film, the air,and the photopolymer and the final image after exposure will bedistorted. If the pockets of air are large enough, they can even lead tomechanical failure of the plate by creating thin spots in thephotopolymer. Moreover, ambient oxygen in any air pockets in contactwith surface of the photopolymer inhibits full curing of thephotopolymer all the way to the photopolymer surface. (This is thoughtto be because oxygen in the air reacts with gases formed within thephotopolymer layer during UV curing, the products of which slow thecuring rate.) Provided the negative and vacuum sheet are skillfullyplaced, the vacuum system in the special exposure unit will remove agreat majority (but not all) such pockets of air. The tops of the halftone dots produced with distortion due to the presence of air arerounded; the printed images of such dots oftentimes appear as tinysquiggles where the dots should be. Even with perfect alignment andapplication of vacuum, single halftone or pixel dots produced this waysuffer from being separated from the photopolymer by the additionalthickness of a cover sheet. They do not have steep shoulders and causedot gain when printed.

BRIEF DESCRIPTION OF THE INVENTION

The laminates and methods of the present invention relate to printing,namely creating fine detail inkjet images that can be placed in directcontact with the surfaces of cast liquid photopolymer to produce flexoplates with “digital quality” flat top dots, that is, single pixel dotshaving a shape like that produced by IR ablation of digital photopolymersheets. The methods further reduce the aforementioned problems with gasbubbles, eliminate layers of material, eliminate the need for expensivee.g. microporous ink-receptive coatings, reduce the number of stepsrequired and the level of expertise necessary to execute them, andproduce final prints of higher quality than achievable by currentmethods under the best of conditions.

Not only do dots created using the laminates and methods of the presentinvention produce a better image, it has also been determined that flexoplates with such digital quality dots last longer (endure moreimpressions) than prior art liquid dots because they distribute theimpact stress more evenly throughout the polymer. All of the above stepsexcept perhaps (c), (h) and (i) require human handling and make theentire process slow, another reason for combining or eliminating some ofthese steps.

Until the applicant's invention described in U.S. patent applicationSer. No. 13/902,301 filed Feb. 19, 2013, it was not possible to createimages for flexo plate making from solvent-based inks that weresufficiently opaque to UV radiation. Only applicant's U.S. patentapplication Ser. No. 14/027,760 filed Sep. 16, 2013, describes andclaims laminates to produce digital quality flat top dots using liquidphotopolymers and solvent-based inkjet inks. This Continuation-In-Partof that application describes and claims other aspects of the samemethods and laminates involving liquid photopolymers.

The first embodiment of the present invention provides for a reliefnegative image to be printed with solvent-based ink onto a vinyl-basedfilm. This film is placed on the lower glass of an exposure unit,preferably image side up, and liquid photopolymer is cast over it. Thesecond embodiment provides for a solvent ink negative image to beprinted on a cover film made of a layer of solvent ink receptivematerial superimposed on a layer of adhesive-releasing material. In thethird embodiment, the solvent ink printable surface may be on a backinglaminate consisting of a flexographic plate backing sheet made of, orcoated with, polyvinyl-based material on its upper surface, with atie-coat applied to the lower surface. The fourth embodiment is abacking laminate consisting of the laminate of U.S. patent applicationSer. No. 14/027,760 printed with solvent-based ink and applied to theupper surface of a layer of liquid photopolymer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a prior art method of usinga liquid photopolymer polymer to make a flexographic plate.

FIG. 2 is a cross-sectional illustration of a prior art method of usinga liquid photopolymer to make a flexographic I-plate.

FIG. 3 is a highly magnified cross-sectional illustration of a singlehalftone or pixel dot produced by the prior art methods.

FIG. 4 is a highly magnified cross-sectional illustration of the priorart dot during printing.

FIG. 5 is a cross-sectional illustration of a solvent ink negativeprinted on a protective cover film in accordance with the firstembodiment of the present invention.

FIG. 6 is a cross-sectional illustration of a solvent ink negativeprinted on a laminate in accordance with the second embodiment of thepresent invention.

(The third embodiment is not illustrated.)

FIG. 7 is a cross-sectional illustration of the present invention,namely the Jetsetter Genesys™ backing laminate of U.S. patentapplication Ser. No. 14/027,760, in accordance with the fourthembodiment of the present invention.

FIG. 8 is a cross-sectional illustration of the method for using theJetsetter Genesys™ backing laminate for making a flexographic I-plate inaccordance with the present invention.

FIG. 9 is a cross-sectional illustration showing removal of the printedcover film and release liner from the finished Jetsetter Genesys™flexographic printing plate made using the fourth embodiment of thepresent invention.

FIG. 10 is a highly magnified cross-sectional illustration of a singlehalftone or pixel dot produced by casting liquid photopolymer over thefirst embodiment printed cover film.

FIG. 11 is a highly magnified cross-sectional illustration of a singlehalftone or pixel dot produced by the first embodiment during printing.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, which are not to scale, and in which likereference characters refer to like elements among the drawings, FIG. 1is a magnified cross-sectional illustration of a prior art method ofusing a liquid photopolymer to make a flexographic plate. An arrangementof plate-making layers 40 is assembled to make a traditionalflexographic plate using a liquid photopolymer where a prior artsubstrate 30 is laminated to the cast liquid. The steps to accomplishthis are:

(a) Place the image negative 27 on the bottom glass 24;

(b) Cover (protect) the negative 27 with a clear cover film 41;

(c) Turn on a vacuum source (not shown) connected to the lower vacuumgrooves 34 to remove air from between the bottom glass 24, the imagenegative 27, and the clear cover film 41;

(d) Depending on the viscosity of the liquid photopolymer, nonporousdamming material 42 may have to be placed around all sides of thenegative 27 to control flow of liquid photopolymer over the negative bycreating a shallow basin on the negative; (Dams are optional and notused in all instances. The liquid photopolymer is very viscous and doesnot readily flow after lamination in the process.)

(e) Pour or cast liquid photopolymer 43 over the cover film 41 (withinthe dams 42 if applicable);

(f) Apply a polyester substrate 30 over the liquid photopolymer 43 withthe tie-coat side down (tie-coat not visible in this view);

(g) Lower the top glass 26 over the entire plate-making arrangement 40;

(h) Turn on a vacuum source (not shown) connected to the upper vacuumgrooves 28 to remove air from between the substrate 30 and the upperglass 26;

(i) Turn on the upper UV lights 52 for a prescribed amount of time toshine through the upper glass 26, causing a uniform floor layer 211 toform in the photopolymer layer 43 below the substrate 30;

(j) Close the shutter (not shown) to prevent light reflection from theupper glass during the image exposure step;

(k) Turn on the lower UV lights 25 to shine through the clear areas 29in the negative 27, causing vertical regions 210 in the liquidphotopolymer 43 above the clear areas 29 to polymerize; and

(l) Remove the plate-making arrangement 40 from the unit for furtherprocessing to create the relief image, namely, by turning theplate-making arrangement over, removing the cover film 41 and dams 42,and draining/collecting the un-polymerized liquid photopolymer forrecycle and reuse.

FIG. 2 is a magnified cross-sectional illustration of a prior art methodof using a liquid photopolymer to make a flexographic I-plate. A priorart plate-making arrangement 60 is assembled to make a flexographicI-plate using a liquid photopolymer where the polyester substrate 30 islaminated to the cast liquid. The steps involved are:

(a) Place the relief image negative 27 on the bottom glass 24;

(b) Cover (protect) the negative with a clear cover film 41;

(c) Turn on a vacuum source (not shown) connected to the lower vacuumgrooves 34 to remove air from between the bottom glass 24, the imagenegative 27, and the clear cover film 41;

(d) Depending on the viscosity of the liquid photopolymer, nonporousdamming material 42 may have to be placed around all sides of thenegative 27 to control flow of liquid photopolymer over the negative bycreating a shallow basin on the negative;

(e) Pour or cast liquid photopolymer 43 over the cover film 41 (withinthe dams 42 if applicable);

(f) Apply a polyester substrate 30 over the liquid photopolymer 43 withthe tie-coat side down (tie-coat not visible in this view);

(g) Place the masking film 51 on top of the polyester substrate 30 andregister it so that the clear areas 53 on the masking film 51 properlysurround the clear areas 29 on the image negative 27 below;

(h) Lower the top glass 26 over the entire plate-making arrangement 60;

(i) Turn on a vacuum source (not shown) connected to the vacuum grooves28 to remove air from between the upper glass 26, the masking film 51,and the substrate 30;

(j) Turn on the upper UV lights 52 to shine through the clear areas 53in the masking negative 51 for a limited time, causing vertical regions54 in the liquid photopolymer below the clear areas 53, adjacent to thepolyester substrate 30, to polymerize a portion of the liquidphotopolymer layer 43 to form island floor areas;

(k) Close the shutter (not shown) to prevent light reflection from theupper glass during the image exposure step;

(l) Turn on the lower UV lights 25 to shine through the clear areas 29in the relief image negative 27, causing vertical regions 210 in theun-polymerized liquid photopolymer above the clear areas 29 (and belowthe vertical regions 54) to polymerize an additional amount of theliquid photopolymer layer 43 to form the relief image; and

(m) Remove the plate-making arrangement 60 from the unit for furtherprocessing to uncover the relief image, namely, by turning theplate-making arrangement over, removing the cover film 41 and dams 42,and draining/collecting the un-polymerized liquid photopolymer forrecycle and reuse.

Note that this plate-making arrangement 60 puts animage-quality-reducing boundary between the masking film 51 and thepolyester substrate 30 which must also be evacuated by the vacuumgrooves 28. Regardless of how well the vacuum works, it is not perfect,and the masking film 51 is displaced away from the liquid photopolymerlayer 43 by the thickness of the polyester substrate 30. (The boundariesbetween the liquid photopolymer layer 43 and the polyester substrate 30above it and the cover film 41 below it are substantially free of airbecause they cover the free surfaces of the liquid.)

FIG. 3 is a more highly magnified cross-sectional view of a portion of aliquid photopolymer layer 43 exposed and developed as described in FIG.1 or 2 under a good vacuum as taught in the prior art. It shows a singlerelief dot 20 formed above a one-pixel-wide transparent area 29 in thefilm negative 27. Even under good vacuum there will be pockets of gas 21(predominantly air) beneath the cover film 41. These variable gaps, inaddition to the thickness of the cover film 41, cause the shoulders 23of the dot to be about 35° off the vertical. When this dot is printed,the edges 50 of the dot 20 become distorted as shown in FIG. 4.

FIG. 4 is a highly magnified view of the same dot 20 as it impacts asheet of paper 44. As the dot 20 is squeezed upward by the printpressure, the dot edges 50 are crushed against the paper 44, increasingthe diameter of the printed dot (“dot gain”) and roughening its edges(reducing its sharpness).

FIG. 5 is a cross-sectional illustration of a first embodiment of thepresent invention, namely a solvent ink negative image 130 printed on animage sheet 131 made entirely of solvent ink receptive material. Thesolvent inks made in accordance with U.S. patent application Ser. No.13/902,301 are satisfactorily printed on cover film material comprisingpolyvinyl-based polymers, particularly polyvinylethylene. Thisembodiment may be used on the bottom glass of a UV exposure unit inplace of a relief image negative covered by a protective cover film,with or without the Jetsetter Genesys™ backing laminate describedfurther below. Using this embodiment, the entire relief image negative27 of the prior art is eliminated, along with the air-containinginterface 21 between the negative and the cover film 41 of the prior art(see FIG. 3). The scope of this invention includes, by way of exampleand not limitation, mixtures or copolymers containing both polyvinylsand polyethylenes, which would be both solvent ink compatible andadhesive releasing, and polyvinyls and polyesters, which would be bothsolvent ink compatible and adhesive retaining.

In this description of the invention, inkjet printers are mentionedbecause they have been used by the inventor to generate the desiredhigh-definition images with solvent-based inks. However, any printercapable of printing solvent-based inks is within the scope of thisinvention without limitation.

FIG. 6 is a cross-sectional illustration of a second embodiment imagesheet of the present invention, namely a solvent ink negative image 130printed on a protective cover film 140 made of a layer of solvent inkreceptive material 131 superimposed on a layer of adhesive-releasingmaterial 141. As in the first embodiment described in FIG. 13, thesolvent inks made in accordance with U.S. patent application Ser. No.13/902,301 are satisfactorily printed on cover sheet material comprisingpolyvinyl-based polymers, particularly polyvinylethylene. The layer ofadhesive releasing material 141 may be comprised of a variety ofpolymer-based films such as polyethylene. The layer of adhesivereleasing material 141 allows this embodiment to be used in place of thecombination of a masking negative and release liner on top of thebacking sheet, and is distinct in this regard from the first embodiment.This cover film/release liner image sheet may be thicker than the coverfilm used in the first embodiment.

Alternatively, as a third embodiment of the present invention (notdrawn) the layer of adhesive-releasing material may be a backinglaminate consisting of a flexographic plate backing sheet made from asolvent ink compatible polymer, with a tie-coat applied to the bottomsurface. This would allow a solvent ink image to be printed directlyonto the backing sheet to be used in the manner of the backing sheettaught and claimed in the aforementioned U.S. patent application Ser.No. 12/895,561.

FIG. 7 is a cross-sectional illustration of the fourth embodiment of thepresent invention. It is a backing laminate called the JetsetterGenesys™ backing laminate 150, ready for solvent-based ink printing. Theunprinted two-layer image sheet 140 of FIG. 6 has been placed againstthe adhesive coating 91 on a polyester backing sheet 31. A solvent-basedimage can now be printed on the solvent ink receptive layer 131.

FIG. 8 is a cross-sectional illustration of the method for using theJetsetter Genesys™ backing laminate of the fourth embodiment togetherwith a relief negative of the first embodiment in making a flexographicI-plate in accordance with the present invention. Steps for preparing anI-plate from liquid photopolymer starting with these two laminates areas follows:

(a) Print the digital representation of the island image 81 on thesolvent ink receptive side of the Jetsetter Genesys™ backing laminate150 (see FIG. 7) using a suitable inkjet printer;

(b) Print the digital representation of the relief image 130 onto thesolvent ink receptive image sheet 131 (see FIG. 5);

(c) Place the image sheet 131 with the relief image 130 on the bottomglass 24 of the UV exposure unit (preferably, but not essentially, withthe image side facing up);

(d) Preferably but optionally, turn on a vacuum source (not shown)connected to the lower vacuum grooves 34 to remove air from between thebottom glass 24, the image negative 27, and the clear cover film 41;

(e) Cast a layer of liquid photopolymer 43 adjacent to the relief imagesheet 131 (employing dams 42 if necessary);

(f) Place the tie-coat 32 of the Jetsetter Genesys™ backing laminate 150in contact with the upper surface of the liquid photopolymer layer 43(registering the island image properly with respect to the relief imagenegative) creating an exposure laminate consisting of, from the bottomup, sheet 131, photopolymer layer 43, and backing laminate 150;

(g) Lower the top glass 26 onto the laminate 150;

(h) Preferably but optionally, turn on a vacuum source (not shown)connected to the upper vacuum grooves 28 to remove air from between thesubstrate 30 and the upper glass 26;

(i) Shine polymerizing light 52 through the Jetsetter Genesys™ backinglaminate 150 for an amount of time suitable to form polymerized islandfloor areas 54 of a desired thickness in the liquid photopolymer incontact with the tie-coat 32;

(j) Shine polymerizing light 25 through the relief image 130 for anamount of time suitable to form a polymerized relief image 210 in theliquid photopolymer between the polymerized island floor areas 54 andthe relief image sheet 131; and

(k) Remove the exposure laminate from the unit for further processing todevelop the relief image, namely, by turning the exposure laminate overand draining/collecting the un-polymerized liquid photopolymer forrecycle and reuse.

Image quality using these laminates is high because, especially with therelief ink image facing up as shown in FIG. 8, both ink-bearing filmsare in intimate contact with both the upper and lower surfaces of thephotopolymer layer. The upper (Jetsetter Genesys™) laminate restsdirectly on the photopolymer layer, and the liquid photopolymer flowsdirectly onto the image surface of the inked surface of the lower coverfilm. (The space 160 in FIG. 8 is a drawing artifact due to theexaggerated vertical dimension of the drawing.)

FIG. 9 is a cross-sectional illustration showing removal of the printedcover film and release liner from the finished flexographic printingplate produced using the fourth embodiment Jetsetter Genesys™. Afterexposure is complete, the lower printed cover film 130 is peeled off ofthe relief image 210 and discarded. The upper printed release liner 111is peeled off of the Jetsetter Genesys™ laminate 150 and discarded orreused. The polymerized island and relief images 54 and 210 respectivelyare cleaned and cured, leaving the finished flexographic printing plate110 with the adhesive layer 91 exposed.

The present invention also includes performing the above method with thealteration of substituting, in step e., for the fourth embodimentbacking laminate 150, solely a polyvinyl-based backing sheet (or otherliquid photopolymer-compatible backing sheet) and tie coat, to produce arelief image only (method not illustrated).

The present invention further includes performing the above method withthe alteration of substituting, in step (f), for backing laminate 150,the two-layer laminate of FIG. 6 with layer 141 being a polyesterbacking sheet with a tie-coat applied to the bottom surface (method notillustrated).

FIG. 10 is a highly magnified view of a relief dot 20 produced using thefirst laminate of the invention as shown in FIG. 5 and prepared inaccordance with the above method of the invention. The cover sheet 131printed with solvent ink image 130 takes the place of the prior artnegative 27 (see FIGS. 1 and 2) and places the image 130 directlyagainst the bottom surface 22 of the liquid photopolymer layer 43. TheUV radiation spreads out much less as it enters the photopolymer than itdoes in the prior art (depicted in FIG. 3). This causes the shoulders 23of the dot to be extended and substantially vertical. When this dot isprinted, the shoulders 23 accommodate print pressure as shown in FIG.11.

FIG. 11 is a highly magnified view of the dot 20 of FIG. 10 as itimpacts a sheet of paper 44. As the dot 20 is squeezed upward by theprint pressure, the dot edges 50 retain their diameter and roundness.

The invention claimed is:
 1. A backing laminate for producing an imagein a liquid photopolymer, comprising: a flexible backing sheet with aliquid photopolymer-binding tie-coat on one side of the flexible backingsheet and an adhesive coating on the other side of the backing sheet; aliquid photopolymer layer applied to the photopolymer-binding tie coat,exposed to actinic radiation, and processed to produce a relief image;and a release liner having one side of the release liner placed againstthe adhesive coating and the other side of the release liner having asolvent ink receptive surface.
 2. The backing laminate of claim 1, inwhich: said solvent ink receptive surface comprises polyvinyl-basedmaterial.
 3. The backing laminate of claim 2, in which: saidpolyvinyl-based material comprises polyvinylethylene.